An example of an application device for dental materials is found in U.S. Pat. No. 5,620,423, which is hereby incorporated by reference. This device includes a plug which has an elastic diaphragm surface. A rotary piston presses the diaphragm surface, and together with the plug, in the direction of an exit nozzle, the material to be squeezed out is put under pressure. This condition of being under pressure is thought to be unfavourable for storage, and therefore in the solution mentioned, the rotary piston is supposed to be rotated in such a fashion that the pressure is released from the material once the squeezing action is finished. Such rotary pistons are considered to be necessary whenever viscous dental materials are to be squeezed out without any particular squeezing device being inserted in the cartridge. In practice, however, the return to stable position is not regularly achieved and/or the rotary piston is sometimes turned in the squeezing direction if the operator does not regularly employ the cartridge concerned.
With viscous, medium viscous and slightly viscous dental materials, problems can arise due to a certain elasticity of the material and also due to a certain “plastic-flow persistence” or incomplete wiping-off, where some dental material may be present outside the tip of the cartridge, which is easily contaminated. Although this material can be wiped off if the device is operated in a careful fashion, in practice, this is not always performed, and thus, this solution is not generally accepted.
Different attempts have been made to overcome the disadvantages of known cartridges. For instance, it has been suggested to provide a special deformation body inside a cartridge upon which shearing forces are exerted and which exerts a resetting impulse. However, dripping of excess material is not prevented with this body in many cases because the shearing deformation is possible only to a very low extent, such that if real drops form, these may drip off if, in particular, materials of a lower viscosity are used.
In order to remedy this problem, it has been suggested to provide a friction guide for the diaphragm by increasing the friction between a diaphragm element and the cartridge wall. For this purpose, the diaphragm element is supported at the actual piston with a predetermined clearance, with the piston extending to the wall of the cartridge, in order to provide possible guidance, even though the diaphragm element has no stability of its own.
In this solution, the diaphragm element has a flange extending to the rear and working with the help of a shoulder pointing inwards against a corresponding shape of the piston rod, such that the clearance can be guided in the desired fashion. By actuating the piston towards the front, the diaphragm element is centrally deformed and is buckled to the front. This deformation, however, is very disadvantageous for the contact with the outer surface, i.e. the contact between the diaphragm element and the cartridge. As a result, the diaphragm element is virtually pulled inside, such that foreign substances, e.g., the dental material to be squeezed out, may easily flow into the clearance between the diaphragm element and the wall, destroying the desired sealing effect.
Although the rear area of the diaphragm element may still be in contact at first, it is not supported by the piston rod, and the virtually wedge-shaped extension of the dental material taken up and being positioned in the margin will be moved towards the rear when the dental material is squeezed out, such that the desired sealing effect is not achieved, which may easily result in the diaphragm element getting stuck or even being destroyed.
It is basically known to seal a piston via a cartridge with a sealing, such as an o-ring sealing, for instance. Here, care must always be taken to prevent any contamination from reducing the sealing function itself, in particular if greater pressures are exerted.